1. Field of the Invention
The present invention relates to a modulator for generating a wide-band angle-modulated signal (phase-modulated signal or frequency-modulated signal), more specifically, through and an optical homodyne detection technique.
2. Description of the Background Art
FIG. 26 is a block diagram showing the configuration of a conventional angle modulator. Operation of such angle modulator is described in detail in documents such as "Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems", K. Kikushima, et al, IOOC 1995 Technical Digest, Vol. 5 PD2-7, pp. 33-34, which is incorporated herein by reference. The angle modulator shown in FIG. 26 includes an optical frequency controller 1000, a signal source 1001, a local light source 1004, an optical modulator 1005, an optical coupler 1006, and a photo-detector 1007.
In the above angle modulator, the signal source 1001 produces an electrical signal, which is an original signal for angle modulation. The optical modulator 1005 is implemented as, for example, a semiconductor laser. In general, the semiconductor laser emits a light having a constant optical frequency f1, provided that an injection current is constant. When the injection current is amplitude-modulated, the optical frequency is also subjected to modulation, and the semiconductor laser emits an optical-frequency-modulated signal centering on the optical frequency f1. With such characteristic, the optical modulator 1005 converts the electrical signal supplied by the signal source 1001 into an optical-frequency-modulated signal for output. FIG. 27B is a schematic diagram illustrating a frequency spectrum of light outputted from the optical modulator 1005.
The local light source 1004 produces an unmodulated light having a constant optical frequency f2. FIG. 27A is a schematic diagram illustrating a frequency spectrum of light outputted from the light source 1004. The optical signal from the optical modulator 1005 and the light from the local light source 1004 are coupled by the optical coupler 1006, and then supplied to the photo-detector 1007.
The photo-detector 1007 is implemented as a photodiode having square-law detection characteristics, for example. The photo-detector 1007 produces a beat signal of the two input lights at a frequency fc equal to the optical frequency difference (=.vertline.f1-f2.vertline.) between the two optical signals L1 and L2. This operation is called optical heterodyne detection.
The beat signal obtained in the above described manner is an angle-modulated signal (frequency-modulated signal) of the carrier frequency fc, and its original signal is the electrical signal from the signal source 1001. FIG. 27C is a schematic diagram illustrating a frequency spectrum of the signal outputted from the optical detector 1007.
The optical frequency controller 1000 controls one or both of the center optical frequency f1 of the optical signal outputted from the optical modulator 1005 and the optical frequency f2 of the light outputted from the local light source 1004 to stabilize the center frequency fc of the angular-modulated signal outputted from the optical detector 1007.
As described above, with the use of high frequency modulation efficiency by optical signal processing (more than ten times the frequency modulation efficiency in ordinary electric circuit systems), the conventional angle modulator can easily generate an extremely high-frequency, wide-band angle-modulated signal (with large frequency deviation or phase deviation), which is difficult to be produced in the ordinary electric circuits.
However, light sources such as semiconductor lasers generally have large phase noise (oscillation spectrum line width), compared with electric oscillators. Phase noise included in the light from the local light source is represented by .DELTA..nu.1 in FIG. 27A, while the phase noise included in the optical signal from the optical modulator is represented by .DELTA..nu.2 in FIG. 27B.
The angle-modulated signal obtained as the beat signal of these two lightwaves has, as shown in FIG. 27C, phase noise equal to the sum of the phase noises of the two lightwaves (.DELTA..nu.+.DELTA..nu.2). The phase noises are simply summed because these lightwaves from the light sources have no interrelationship in phase each other. When the angle-modulated signal is demodulated, the phase noise is also demodulated and becomes large white (intensity) noise. This white noise produces serious deterioration of a demodulated signal in quality.
Further, the conventional angle modulator has to always control and adjust the optical frequencies of the signals from the two light sources (or the difference therebetween). Therefore, the conventional angle modulator requires a control circuit for control and adjustment, for example, resulting in a complicated configuration.